1. Field of the Invention
The present invention relates to a binder having a specified composition, which is added to a powder metallurgical raw powder, mainly containing a metal powder such as iron powder or steel powder incorporated with alloy powders and graphite as components of improving the physical properties and a lubricant powder such as zinc stearate, for suppressing the segregation of the physical property improving powders and the lubricant powder without degradation of the physical properties of the metal powder as the main component, suppressing the generation of dust in handling of powders, and suppressing the deterioration of the lubricity.
2. Description of the Related Art
In manufacturing sintered structure parts, etc. from a powder metallurgical raw powder mainly containing a metal powder such as iron powder or steel powder, the raw powder is usually added with alloy powders such as copper, nickel, chromium, molybdenum and powders of graphite, phosphorus and sulfur for improving the physical properties (strength characteristics, machinability, etc.) of the sintered body, and a lubricant powder such as zinc stearate. However, the above physical property improving powders and the lubricant powder are usually very different in particle size and specific gravity from each other. For example, where the main metal powder is iron powder or steel powder and the physical property improving components are graphite and phosphorus, they are significantly different in specific gravity from each other, which tends to generate dust and segregation in the handling step until compaction after mixing. To solve this problem, various attempts have been made for a long time.
The generation of dust is mainly due to fine powders having small specific gravities such as graphite powder, and it not only causes an environmental problem in handling of powders but also reduces the yield. The segregation tends to be generated where powders different in specific gravity and particle size from each other are mixed. For example, it is well known that when a mixed powder is discharged from a hopper, the mixing ratio of alloy powders is changed depending on the elapsed time during the discharge by the effect of the segregation.
To prevent the generation of dust and segregation, various methods have been proposed. These are generally classified into the following three types: The first method involves the step of adding a liquid additive such as tall oil to a raw powder, as is disclosed in Unexamined Japanese Patent Publication No. SHO 60-502158. The second method involves the step of dissolving a solid binder with solvent and uniformly mixing it with a raw powder, and then evaporating the solvent, as is disclosed in Unexamined Japanese Patent Publication Nos. SHO 63-103001 and HEI 2-217403. The third method is the so-called hot melt process in which a solid binder is melted in mixing with a raw powder, as is disclosed in Unexamined Japanese Patent Publication No. HEI 1-219101. In particular, the second method is known to be excellent in improvement of the adhesive strength of graphite powder and in wide selection of the kind of lubricant.
These methods are all excellent in preventing the generation of dust and segregation, however, they have the following disadvantages: The first method is disadvantageous in that the angle of repose of a mixed powder is increased, to deteriorate the flowability, thus easily generating the so-called bridging phenomenon upon the discharge of powders from a hopper. Where graphite powder is deposited on the surface of each particle of a metal powder such as iron powder, the flowability of the mixed powder is improved, but the lubricity of the graphite powder is deteriorated so that the friction between a die and powders or between powders upon compaction is increased, thereby deteriorating the lubricity of the mixed powder compared with the usual mixed powder. The second and third methods using a solid binder are disadvantageous in that the decomposition of the binder is often made poor, that is, the decomposition of the binder in the dewaxing process is made insufficient, with a result that the remainder possibly presents in a sintered body.
The conventional graphite segregation preventive powder is capable of preventing the generation of dust and segregation of graphite powder and having a flowability being not poor so much; however, it is inconvenient in deteriorating the lubricity of the graphite powder, thereby failing to achieve the good lubricity of the mixed powder upon compaction.